The present invention relates to a fluid treatment module, notably for ultra-pure fluids, using separation or catalytic transformation, the module having at least one treatment element of the rigid membrane type. The invention also relates to treatment processes using this module. The separation can for example be a liquid filtration using, for instance, microfiltration, ultrafiltration, nanofiltration, or reverse osmosis or, yet again, gas filtration through one or more membranes. The catalytic transformation can be, for example, reaction of the fluid with a catalyst located on or in a membrane, or, yet again, the reaction of the fluid circulating at one side of a membrane with a second fluid originating from the other side of this membrane and through which this second fluid passes, the function of the membrane then being to regulate the flow of the second fluid.
A membrane is a separating element which generally comprises three parts:
a rigid support having crude porosity and being highly permeable, imparting shape and rigidity to the element. It generally consists of a porous sintered material which can be a ceramic or a metal, or yet again, carbon; PA1 a very thin active layer deposited on the support surface, providing the separating function. This active layer, which is porous in the case of a filtration, covers the inner surface, or channels, of the separator element, or, in some case, the outer surface when tubes are used. It can be deposited on the support, or on one or several sub-layers themselves deposited on the support; PA1 a "sealant" filling the porosity at each end of the support, so as to prevent communication between the upstream and downstream regions of the module, via the crude porosity of the support, where the active membrane does not cover the whole surface of the end of the support. Membranes of the multi-channel type are for example described in the publication "New ceramic filter media for crossflow microfiltration and ultrafiltration, J. Gillot, G. Brinkman, D. Garcera, Fourth World Filtration Congress, Apr. 22-24, 1986, Ostend, Belgium. PA1 one or several separation or catalytic transformation rigid membranes generally of an elongated shape, usually tubular or prismatic; PA1 a casing designed to hold the fluid to be treated and which the membrane or membranes separate(s) into several cavities one of which at least contains the fluid to be treated, one of which at least containing the treated fluid. PA1 a tube housing the axis of which is parallel to the axis of the tubular or multitubular membranes and which includes lateral orifices for entry of the fluid to be treated or evacuation of treated fluid; PA1 two end plates perpendicular to the cylinder axis and which close off the two ends thereof. These plates carry at least one, and, generally, several orifices, each receiving a seal or gasket for a membrane, the fluid to be treated, or treated fluid, flowing through these orifices; PA1 mounting seals or gaskets located between the membranes and the wall of the orifices of the end plates, these seals ensuring the membranes are held in place and providing inter-cavity sealing inside the tube housing of the casing. PA1 a casing of a rigid material compatible with the fluid to be treated, PA1 a compensating plate at at least one end of said casing, said compensating plate being dimensioned so as to slide longitudinally in the end of said casing adapted to receive it, and carrying one or several orifices each adapted to receive one end of a membrane via an assembly seal comptible with a fluid to be treated, said compensating plate being made of a material which is compatible with a fluid to be treated, and having a coefficient of expansion identical or very close to a coefficient of expansion of said casing, PA1 a compression plate at one or each end of said casing and associated with each compensating plate and being applied against said compensating plate while maintaining the possibility of sliding sideways, the or each compression plate being highly rigid and consisting of a material of great mechanical strength coated with a material compatible with a fluid to be treated at least in the regions where said fluid may circulate, said orifices facing one or more orifices in said compensating plate against which said compression plate is applied, PA1 tie rods passing through holes provided at the periphery of the or each compression plate, the or each compression plate exerting pressure on the end or ends of the membrane or membranes via said compensating plate upon which said compression plate bears thereby ensuring said membrane or membranes are held in position inside said casing, under the action of said tie rods when said tie rods are mechanically tensioned.
Here, the term "module" means an assembly comprising:
The casing comprises, as is notably shown in FIG. 1 of European Patent EP-0,270,051:
Such modules, which are known, can comprise metal or ceramic membranes mounted inside a metallic casing, typically of stainless steel, along with at least seals made of elastomer material (for example EPDM, silicone, fluoroelastomer) as for example disclosed in EP-0,270,051 or EP-0,385,089.
In the case of ceramic membrane modules, the casing is typically a stainless steel cylinder inside which between one and hundred tubular or multi-tubular membranes, about one meter long, are mounted between elastomer seals.
However, such modules are not suitable for treating certain highly pure-fluids such as for example ultra-pure water. Indeed, their stainless steel casing releases metal ions which decrease the purity of the treated fluid, notably in the case of ultra-pure water, which can be particularly harmful when this water is employed in the electronic component manufacturing field, or in the pharmaceutical field. Similarly, it is not possible to treat corrosive fluids, and notably fluids containing chlorine ions, using the modules as described above.
However, materials referred to herein as "compatible materials" do exist which are inert vis-a-vis the fluids with which they may come into contact, for example in a module, to the extend that, firstly, there is no risk of them being corroded by the fluid or fluids, and secondly, they will not contaminate the latter. As is known, such materials can be more or less suited to one given fluid, and a suitable one should be chosen as a function of the fluid or fluids concerned. However, it cannot be envisaged to provide modules implementing such materials by simply substituting one compatible material by a material used up until now in known modules, because of the differences in behavior between materials.
Indeed, materials which are compatible, in the sense succinctly given above, with ultra-pure fluids, such as certain polymers and, for example, poly (vinylidene fluoride) (PVDF) have a coefficient of expansion which is much higher than that of ceramics or materials employed for constituting rigid membranes. Conventional assemblies using elastomer seals do not make it possible to compensate this difference in expansion when temperature variations to which the module is subject exceed some ten degrees, which is too small for numerous applications to allow the module to be used in practice.
Moreover, prior art stainless steel modules do not either make it possible to treat certain corrosive fluids such as, for example, liquids containing chlorine ions, which corrode stainless steel. It is then necessary to use casings in special materials such as titanium alloys, which are very expensive.
In order to nevertheless construct a module along conventional lines, and which does not discharge metal ions or which can treat a large variety of corrosive liquids, one can envisage coating all the inner surfaces of a conventionally designed module having a stainless steel casing, with a suitable polymer such as PVDF. However, with this solution, the orifices of the end plate have sharp edges which are difficult to coat uniformly. Moreover, in order to fulfil their function of maintaining the membranes, the elastomer seals are subject to considerable forces which can cause the polymer coating deposited on the orifice wall to slide, or the coating to be damaged. Finally, such a casing consisting of polymer-coated steel would be more expensive due to the complexity of the operation of depositing a coating on surfaces of complex shape.